1. Field of the Invention
The present invention relates to a liquid crystal display device, more particularly, a liquid crystal display device containing a ferroelectric liquid crystal material sandwiched between first and second electrodes. The display device is driven in accordance with a simple matrix address system, and therefore, can provide a large area and high information content display, high contrast display and high gradation display. Further, since the display device contains a dielectric layer as a capacitative element disposed and electrically connected therein in parallel to the liquid crystal material with regard to an external electric source, a stabilized memory effect can be obtained as a result of an extension of a relaxation time of the spontaneous polarization of the liquid crystal material. Therefore, the display device can be widely and advantageously utilized as a display panel in word processors, personal computers, work stations and other devices.
2. Description of the Related Art
Liquid crystal display devices are widely used as a display panel in word processors or laptop computers, due to their flat panel configuration and are drivability at a low power consumption. Particularly, the display devices using a super twisted nematic (STN) liquid crystal are more widely used and are especially suitable as a display panel for large display size devices such as personal computers, since large size devices having a display area of 12 inches diagonal can be relatively easily produced using the STN liquid crystals.
Recently, a need has arisen for an increased display speed, in connection with an increased display area or information content amount of the display panels, as the display panels are now applied to high-performance word processors and personal computers. The above-described STN liquid crystals, however, cannot solve this problem. Practically, the STN liquid crystals are advantageous when displaying character information, as in word processors, but cannot be used to display a moving picture due to an unacceptably slow display speed.
More recently, it has been found that ferroelectric liquid crystal devices (FLCDs) or liquid crystal display devices using the ferroelectric liquid crystal material can exhibit a remarkably increased display speed, in addition to the many advantages of the STN devices described above. These FLCDs are described in, for example, N.A. Clark and S. T. Lagerwall, "Submicrosecond Bistable Electro-Optic Switching in Liquid Crystals", Apple. Phys. Lett. 36 (11), 899, Jun. 1, 1980. Note, a ferroelectricity of the FLCDs is based on a spontaneous polarization of the liquid crystal molecules, and the ferroelectricity occurs when the spontaneous polarization is reversed as a result of the reversal of the polarity of the applied electric field.
Generally, as is well-known in the art, the FLCDs comprise a layer of the ferroelectric liquid crystal sandwiched between the opposed first and second electrodes. To drive the FLCDs, pulsed voltage or electric field with a positive or negative polarity is applied to the FLC layer through the first and second electrodes from an external electric source. The spontaneous polarization of the liquid crystal molecules is thus adjusted to the same direction, and this state, i.e., information display state, is maintained by the ferroelectricity of the liquid crystal, i.e., the memory effect of the liquid crystal in which the direction of the spontaneous polarization is not changed. When the displayed information is to be changed, it is possible to newly apply a pulsed voltage having a reversed polarity to the FLC layer, thereby changing a direction of the spontaneous polarization therein. Apparently, the performances of the resulting FLCDs depends on how long a memory effect of the liquid crystal can be stably maintained.
An equivalent circuit of the prior art FLCDs is illustrated in, for example, FIG. 1. The FLC device 1 comprised a capacitor 2 with the capacitance C.sub.LC and a resistance 3 having a sum R of the internal resistances of the device 1 and an external electric source 4, and is connected to the electric source 4. As described hereinafter, the FLCDs suffer from a relaxation of the spontaneous polarization thereof, which relaxation corresponds to a discharge of the electric charge stored in the capacitor of the device. The relaxation time .tau. is calculated in accordance with the following equation: EQU .tau.=k.multidot.R.times.C.sub.LC ( 1)
in which k is a proportionality factor. In use, the prior art FLCDs having the equivalent circuit such as that of FIG. 1 show a reduction of transmittance with time. For example, when a pulsed voltage having a predetermined level is applied at 400 .mu.s to the FLCD, as shown in FIG. 2, information is displayed made at a predetermined transmittance. The displayed information should be maintained without reduction of the transmittance, until the next pulsed voltage having a reversed polarity is applied to the device (0.5 s). As shown in the graph of the transmittance of FIG. 2, however, the transmittance is significantly reduced within 0.5 seconds, and this significant reduction of the transmittance means a reduction of the contrast and other performances of the device. Note, of course, a minor reduction of the transmittance cannot be detected by the naked eye.
The above-described reduction of the transmittance of the FLCDs with time is considered to be due to a change of the direction of the spontaneous polarization of the liquid crystal; ideally such a change does not occur in the FLCD3. This will be further described with reference to FIGS. 3A and 3B, which correspond to FIG. 1.
First, to drive the FLC device 1, a pulsed voltage is applied to the device 1 from an external electric source 4. As shown with arrows in FIG. 3A, the liquid crystal molecules of the device have the same direction of spontaneous polarization. The illustrated direction of the spontaneous polarization should be stably maintained during the display period of the information, to thereby ensure a stabilized memory effect of the FLC device. Nevertheless, the FLC device 1 is liable to gradually and partially change the direction of the spontaneous polarization.
An undesirable change of the spontaneous polarization is shown in FIG. 3B (see, three dotted lines of the device 1). More particularly, although not shown in FIG. 3B, the direction of the spontaneous polarization is gradually shifted to the left or right, and in some cases, is completely reversed. The reasons for this change are, for example, nonuniform orientation of the liquid crystal, inconsistency of the interaction between the interfacial substrate and the liquid crystal molecules, and an incomplete structure of the liquid crystal layer.
Since a closed circuit as shown in FIG. 3B is produced after the application of the pulsed voltage, the device will be metastable. The metastable state is created by the generation of the internal electric field in the device due to the spontaneous polarization of the liquid crystal itself. The generated internal electric field will act to negate as external electric field applied to the device. Thus, the internal electric field makes a depolarization of the electric field. Accordingly, as times passes, the direction of the spontaneous polarization of the liquid crystal is partially switched for the above-described reasons, e.g., nonuniformity of the orientation of the liquid crystal. When the direction of the spontaneous polarization is switched, the internal electric field is reduced or damped in the same sites of the device, and therefore, induce a reversal of the spontaneous polarization of the liquid crystal in other sites of the device, and thus the internal electric field is gradually reduced as a whole and eventually reaches zero. The absence of the internal electric field means that the memory capability of the device has been lost.
Under these circumstances, there is a need to provide an improved liquid display device having an excellent memory capability sufficient to ensure a stable drive of the device, and this is one object of the present invention. Note, in the prior art, it is impossible to completely remove the nonuniformity of the orientation and other factors in the production of a large size liquid crystal panel having a display area of, for example, 12 inches diagonal, although it is obvious that the above-described causes, including the nonuniformity of the liquid crystal panel, must be eliminated from the FLC device, to prevent the relaxation of the spontaneous polarization of the liquid crystal and thereby stably maintain the memory capability of the device.
Furthermore, although the object thereof is not a stable maintaining of the memory capability of the device, another type of the liquid crystal display device is well-known which is addressed by a nonlinear switching element such as thin-film transistor (TFT) and is briefly referred to as a TFT-LCD. The liquid crystal display mode of the TFT-LCDs is the twisted nematic (TN) mode, and a drive method thereof is an active matrix addressing. A principal object of these devices is to stably maintain the electric charge generated in the transistor stably, but TFT-LCDs can provide high quality images comparable to those of CRT (Cathode Ray Tube). The TFT-LCDs are described in, for example, M. Ikeda et al., Low Resistance Copper Address Line for TFT-LCD, Japan Display '89, pp. 498-501.
An equivalent circuit of the TFT-LCDs is illustrated in FIGS. 4 and 5. The device 5 comprised a capacitor 6 of the LC material with the capacitance C.sub.LC and a TFT 7 having the internal resistor R.sub.LC, as well as a capacitor 8 with the capacitance C.sub.s. The capacitor 8 is a memory storage capacitor. As apparent from these drawings, the TFT 7 is fabricated on each picture element (see also FIG. 6), and it and the LC material 6 are connected in parallel with regard to an external electric source (not shown). To ensure a stable retention of the voltage generated in the transistor, the capacitor or LC material 6 must have a capacitance C.sub.LC which conforms to the size of the picture element.
A typical constitution of the TFT-LCDs is illustrated in FIG. 7. A light source 11 is a fluorescent lamp from which the light is guided, in sequence, through a light guide 12 and a polarizer 13 to the TFT-LCD. As illustrated, a glass substrate 14 of the device has a TFT fabricated thereon. The TFT contains a gate electrode 15, source area 17, and drain area 18, as well as a display electrode 16 of indium-tin oxide (ITO). Another glass substrate 22 of the device has a black matrix 21, color filter 20, and counter electrode 19 of ITO coated in sequence thereon. Another polarizer 23 is disposed over the substrate 22. A twisted nematic (TN) liquid crystal 10 is sandwiched between the electrodes 16 and 19. Note, a gate insulating layer is not connected through an electrically conductive means by an opposed electrode 19 in the illustrated TFT-LCD, but as described hereinafter, a dielectric layer is connected through the electrically conductive means to the opposed electrode for the FLCD of the present invention.